Conversion coatings for metal surfaces

ABSTRACT

An aqueous treatment solution for producing conversion coatings on metal surfaces, in particular on zinc or zinc alloy surfaces, contains: Cr(III) ions in an amount of 0.1 g/l to 8.0 g/l; zirconium and/or titanium ions in an amount of 0.1 g/l to 15 g/l; organosilane-modified silicon oxide nanoparticles in an amount of 0.1 g/l to 50 g/l; and fluoride ions in an amount of 0.1 g/l to 15 g/l.

CROSS-REFERENCE TO PRIOR APPLICATIONS

This application is a U.S. National Phase application under 35 U.S.C. § 371 of International Application No. PCT/EP2017/000560, filed on May 5, 2017, and claims benefit to German Patent Application No. DE 10 2016 005 656.8, filed on May 11, 2016. The International Application was published in German on Nov. 16, 2017 as WO 2017/194187 under PCT Article 21(2).

FIELD

The invention relates to a treatment solution and a process for producing conversion coatings on metal surfaces, particularly zinc or zinc alloy surfaces, using this treatment solution. The invention further relates to a concentrate for manufacturing the treatment solution and conversion coatings, which can be produced by means of the process in accordance with the invention. The treatment solution in accordance with the invention allows metal materials to obtain a high level of corrosion protection, with the decorative and functional properties of the surfaces simultaneously being achieved or improved. Additionally, it is also possible to avoid known problems when using heavy metal ions, particularly cobalt and chromium(VI) ions.

The invention relates to the corrosion protection of metal materials, particularly of zinc-containing surfaces.

BACKGROUND

In prior art various methods for protecting metal material surfaces from corrosive environmental influences are provided. A widely used and established method in prior art is the application of a metal coating to the metal material to be protected. In this way, materials made of iron and steel are for example often galvanized or cadmium-plated, to protect them from corrosive environmental influences. As a result, the coating metal may behave in the corrosive medium in an electrochemically more noble or less noble way than the base metal of the material on its own. If the coating metal behaves in a less noble way, it acts in the corrosive medium in line with a cathodic corrosion protection against the base metal as a sacrificial anode. Thus, the corrosion protection of the zinc is based on the fact that it is even less precious than the base metal and therefore it initially exclusively attracts the corrosive attack. Although this protective function is desirable, the corrosion products of the coating often lead to undesirable impairments of the decorative and often also the functional properties of the material.

In order to reduce the corrosion of the coating metal or to prevent it as long as possible, it is common to use so-called conversion coatings especially on cathodically-protecting base coating metals, such as zinc and its alloys. These are reaction products of the base coating metal with a so-called treatment solution, which are insoluble in aqueous media in a wide pH range. Examples for these so-called conversion coatings are so-called phosphatings and chromatings. In the case of phosphatings, the layer to be protected is immersed in an acidic solution containing phosphate ions. The acidic medium results in a partial dissolution of zinc from the coating. The released Zn2+ cations form together with the phosphate ions of the reaction solution form a sparingly soluble zinc phosphate layer on the surface. Since zinc phosphate layers themselves only provide comparatively poor corrosion protection but are an excellent adhesion base for paints and varnishes applied to them, their main application is as a primer for paintings and lacquerings.

In the case of chromating treatments, the surface to be treated is immersed in an acidic solution containing chromium(VI) ions. By applying a chromate coating, the corrosive attack on the coating metal can in turn be greatly delayed and thus the base metal corrosion can be further delayed with respect to applying the coating metal alone. Furthermore, the optical impairment of a component due to environmental influences is also delayed by chromating. The corrosion products of the coating metal (in the case of zinc the so-called white rust) also have a disturbing effect on the appearance of a component.

However, chromium(VI) compounds have the disadvantage that they have a high carcinogenic potential in addition to their acute toxicity. As a replacement for chromating processes using hexavalent chromium compounds, a large number of processes have been established in the meantime that use, among other things, various complexes of trivalent chromium compounds.

However, the methods currently used in practice generally use compounds classified as SVHC (Substances of Very High Concern), such as cobalt ions, to achieve the required corrosion protection.

For example, EP 1346081 A1 describes a process for passivating zinc, cadmium or their alloys, especially zinc-nickel alloys, with a chromium(VI)-free solution containing a weak complexing agent, preferably di- or tricarboxylic acids, preferably chromium(III)oxalate complex and Co2+, in which the Co2+ concentration is more than 30 g/l.

However, cobalt ions are to be classified as harmful to health and their use is restricted by various regulations or is listed as a candidate (such as REACH).

DE 19615664 describes a chromate-free, essentially coherent chromate layer on zinc and zinc alloys, in which Cr(III) is an essential component of the layer and which, already without the presence of silicate, cerium, aluminum and borate, has a corrosion protection of about 100-1000 h according to the salt spray test DIN 50021 SS (ISO 9227) or ASTM B 117-73 until initial corrosion attack as defined in DIN 50961, Chapter 10. The conversion coating is clear, transparent, colorless and has a greenish-multicolored iridescence, is about 100 nm to 1000 nm thick, hard, adhesive and smudge-proof.

The use of fluoride is described as disadvantageous in this publication because the exchange kinetics of fluoride as a complex ligand of the chromium used is too slow and the layer cannot achieve the desired thickness as a result.

However, the use of fluoride as a complexing agent in conversion baths is basically desirable as it is easy to use in the application and analytically controllable.

SUMMARY

In an embodiment, the invention provides an aqueous treatment solution for producing conversion coatings on metal surfaces, in particular on zinc or zinc alloy surfaces, the treatment solution containing: Cr(III) ions in an amount of 0.1 g/l to 8.0 g/l; zirconium and/or titanium ions in an amount of 0.1 g/l to 15 g/l; organosilane-modified silicon oxide nanoparticles in an amount of 0.1 g/l to 50 g/l; and fluoride ions in an amount of 0.1 g/l to 15 g/l.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be described in even greater detail below based on the exemplary figures. The invention is not limited to the exemplary embodiments. Other features and advantages of various embodiments of the present invention will become apparent by reading the following detailed description with reference to the attached drawings which illustrate the following:

FIG. 1: Cross-section of a conversion coating produced using the treatment solution 3 in accordance with the invention

FIG. 2: Cross-section of a conversion coating produced using the treatment solution 4 in accordance with the invention

DETAILED DESCRIPTION

The object of the invention is to provide a treatment solution of the kind mentioned above, which allows to avoid at least some of the disadvantages mentioned. In particular, the treatment solution should be able to produce conversion coatings on metallic surfaces, in particular zinc or zinc alloy surfaces, which offer high corrosion protection and at the same time can be free of cobalt and chromium (VI) compounds. In particular, the treatment solution should be able to be free of intentionally added cobalt and chromium(VI) compounds. In addition, it should be possible to use fluoride and still achieve a sufficient layer thickness.

This object is achieved by an aqueous treatment solution for the generation of conversion layers on metallic surfaces, especially on zinc or zinc alloy surfaces, where the solution contains:

-   -   Cr(III) ions in an amount of 0.1 g/l to 8.0 g/l     -   Zirconium and/or titanium ions in an amount of 0.1 g/l to 15 g/l     -   Organosilane-modified silicon oxide nanoparticles in an amount         of 0.1 g/l to 50 g/l     -   Fluoride ions in an amount of 0.1 g/l to 15 g/l.

Surprisingly, it was found in accordance with the invention that the treatment solution makes it possible—despite the use of fluoride ions—to produce conversion coatings with a high coating thickness of more than 100 nm. This is also possible without the use of solutions with high concentrations or high treatment temperatures.

Treatment solutions in accordance with the invention can be produced low in or essentially free of volatile organic compounds (VOCs) and are therefore environmentally and/or climate-friendly.

In accordance with the invention, it was also found that the treatment solution in accordance with the invention makes it possible to dispense with the use of metal ions harmful to health, in particular cobalt and chromium(VI) compounds, and still provide conversion coatings on zinc or zinc alloy coatings which offer excellent corrosion protection. Practical tests have shown, for example, that zinc or zinc alloy coatings can be provided with corrosion protection for more than 240 hours by treating them with a treatment solution in accordance with the invention, measured in the salt spray test according to ISO 9227 and/or ASTM B 117-73, even after heat stress, for example at 120° C. for 24 hours until initial attack according to DIN 50961 Chapter 10, of more than 240 hours.

In a preferred embodiment of the invention, the treatment solution in accordance with the invention is essentially free of cobalt and chromium(VI) compounds. Thus, the treatment solution preferably contains less than 1 mg/l, preferably less than 0.8 mg/l, in particular less than 0.6 mg/l of chromium(VI) ions. The proportion of chromium(VI) ions can be determined photometrically using diphenyl carbazide. According to another preferred embodiment of the invention, the solution contains less than 10 mg/l, preferably less than 5 mg/l, in particular less than 2 mg/l of cobalt ions. Thereby, the proportion of cobalt ions, measured as cobalt, can be determined using ICP.

Furthermore, it is also possible to dispense with Ni ions in the passivation solution. For example, the Ni ion content in one embodiment of treatment solution is preferably less than 10 mg/l, even more preferably less than 5 mg/l, in particular less than 2 mg/l of nickel ions. Thereby, the proportion of nickel ions, measured as nickel, can be determined by means of ICP.

The term zinc or zinc alloy surfaces is understood in the conventional sense. This includes in particular surfaces of articles such as zinc sheet, zinc die casting, zamac, galvanized steel. Zinc alloys can contain up to 30% of foreign metals such as aluminum, iron and nickel.

In accordance with the invention, the passivation solution contains Cr(III) ions in an amount between 0.1 g/l and 8.0 g/l. Thereby, the proportion of Cr(III) ions, measured as chromium, can be determined by ICP.

The advantage of chromium(III) is that it is less dangerous than chromium(VI). As a chromium(III) oxide in the conversion layer, it can provide a particularly good barrier against corrosive external influences, as it is chemically inert.

In accordance with the invention, the passivation solution also contains zirconium and/or titanium ions in an amount between 0.1 g/l and 15 g/l. The proportion of zirconium and/or titanium ions, measured as zirconium and/or titanium, can be determined using ICP.

The advantage of using zirconium and/or titanium ions is that they are easily precipitable in combination with chromium. This allows the layer thickness to be further increased, whereby the weight content of zirconium and/or titanium in the conversion layer produced can even be higher than the chromium content.

As a further component, the treatment solution in accordance with the invention contains organosilane-modified silicon oxide nanoparticles. In these nanoparticles, the silicon oxide is preferably present in the form of a nanoscale agglomerate. This agglomerate can be regarded as a core whose surface is silane-modified, i.e. on whose surface organic silane compounds are arranged. Organosilane modification means in particular that oxygen atoms are covalently bonded to silicon atoms of an organic silicon compound at least on the surface of the silicon oxide nanoparticles. The organic silicon compound is preferably an epoxy, amido, ureido, amino, ester, mercapto and/or isocyanate silane.

It is conceivable that different silicon oxide nanoparticles are modified with a different number and/or type of organic silicon compounds. Silicon oxide nanoparticles may also contain various silicon compounds in combination. The stoichiometric composition of the silicon oxide nanoparticles may vary depending on the manufacturing process.

The organosilane-modified silica nanoparticles are present in the treatment solution in an amount between 0.1 g/l and 50 g/l in accordance with the invention. These concentration values refer to the total solids' concentration of the organosilane-modified silicon oxide nanoparticles in the treatment solution.

The organosilane-modified nanoparticles preferably have an average particle size of 5 to 50 nm. The nanoparticles are advantageously at least partially dispersed in the treatment solution in accordance with the invention. Such nanoparticles can, for example, be produced as described in EP 2406328 A1. In addition, suitable nanoparticles are commercially available, for example under the brand name Bindzil® from the company Akzo.

The advantage of using organosilane-modified silicon oxide nanoparticles is that they have a high stability even at acid pH values. Another advantage is that the silane modification slows down an attack on the silicon oxide nanoparticles by the fluoride ions used in accordance with the invention.

In addition, the silicon oxide nanoparticles increase the resistance of the conversion coating.

The treatment solution according to invention contains fluoride in a quantity between 0.1 g/l and 15 g/l. Thereby, fluoride is understood as both free and complex bound fluoride. The concentration value refers to the total fluoride content.

The use of fluoride in conversion baths is advantageous because it is easy to use and can be controlled analytically, for example by ion-selective potentiometry. In addition, fluoride is advantageous for the dissolution of zirconium and/or titanium components because the complexes formed are particularly stable in aqueous environments.

The proportion of components contained in the treatment solution may vary depending on the desired performance characteristics. In practical experiments, it has proved to be particularly favorable if the content of:

-   -   Cr(III) ions in the treatment solution is from 0.15 g/l to 6         g/l, particularly preferred from 0.2 to 5 g/l and/or of     -   Zirconium and/or titanium ions is from 0.15 g/l to 10 g/l,         particularly preferably from 0.2 g/l to 8 g/l and/or of     -   organosilane-modified silicon oxide nanoparticles is from 0.2         g/l to 40 g/l, particularly preferably from 0.3 g/l to 30 g/l         and/or of     -   fluoride ions is from 0.15 g/l to 15 g/l, particularly         preferably from 0.2 g/l to 10 g/l.

It has also been shown to be advantageous if the molar ratio between Cr(III) ions on the one hand and zirconium and/or titanium ions on the other hand in the treatment solution is 0.1 to 2.0, preferably 0.15 to 1.5, particularly preferably 0.2 to 1.

Water-soluble chromium (III) salts such as chromium chloride (CrC13), chromium nitrate (Cr(NO3)3), chromium sulfate (Cr2(SO4)3), chromium fluoride (CrF3), chromium methane sulfonate, MSA (Cr2(CH3SO3)3) are preferably used as the source for the chromium (III) ions.

For example, the hexafluoro complexes of zirconium and/or titanium as acid and/or their salts can be used as source for the fluoride and simultaneously zirconium and/or titanium ions.

In addition to fluoride, other complexing agents can also be used, such as the various complexing agents commonly used for generic treatment solutions. Particularly good results are achieved in accordance with the invention with complexing agents selected from the group consisting of: Carboxylic acids, in particular formic acid, acetic acid, acetylsalicylic acid, benzoic acid, nitrobenzoic acid, 3,5-dinitrobenzoic acid; amino acids, in particular alanine, arginine, asparagine, aspartic acid, cysteine, glutamine, glutamic acid, glycine, histidine, isoleucine, leucine, lysine, methionine, phenylalanine, proline, serine, threonine, tryptophan, tyrosine, valine, chelating ligands, in particular dicarboxylic acids, tricarboxylic acids, hydroxycarboxylic acids, in particular oxalic, lactic, malonic, succinic, glutaric, adipic, pimelinic, cork, azelainic, sebacic acids; further, hydroxypolycarboxylic acids, maleic acid, gluconic acid, phthalic acid, terephthalic acid, tartaric acid, citric acid, malic acid, ascorbic acid; and further ligands such as acetylacetone, urea, urea derivatives, and further complex ligands in which the complexing functional group contains nitrogen, phosphorus or sulfur (—NR2, —PR2, where R is, independently of one another, an organic, in particular C1 to C5 aliphatic radical and/or H, and/or —SR, where R is an organic, in particular C1 to C5 aliphatic radical or H); phosphinates and phosphinate derivatives; and suitable mixtures thereof. The other complexing agents, if present, are preferably present in a concentration between 0.0 g/l and 25 g/l, and/or between 0.1 g/l and 25 g/l.

Other chelating agents particularly preferred in accordance with the invention are chelating ligands, for example suitable carboxylic acids, in particular dicarboxylic acids, tricarboxylic acids, hydroxycarboxylic acids, and/or amino acids as well as suitable mixtures thereof.

Preferably the complexing agent is selected such that the Cr(III) ions in the treatment solution are at least partially complexed by the complexing agent. The advantage of this is that the process solution is stable in a wide pH range, in particular from pH 1 to pH 5, and the optical appearance of the conversion layer improves and is homogenized.

The treatment solution may also contain other metal or metalloid ions. Thus it is conceivable that it contains at least one further metal or metalloid ion selected from the group consisting of Na, Ag, Al, Co, Ni, Fe, Ga, In, Lanthanides, Sc, V, Cer, Cr, Mn, Cu, Zn, Y, Nb, Mo, Hf, Ta, W, B, Si, P, Bi, Sb, Se ions, preferably in a concentration between 0.005 and 5 g/l.

These metal ions can, for example, act as catalysts and are added to the reaction solution preferably as soluble salts, in particular as nitrates, sulphates or halides.

In addition, the treatment solution may also contain other anions commonly used in generic treatment solutions. Thus it is conceivable that they contain at least one anion selected from the group consisting of halide ions, in particular chloride, iodide, sulphur-containing ions, in particular sulphate ions, methane sulphonate, hydrogen sulphate; nitrate ions; phosphorus-containing ions, in particular phosphate ions and anions of esters of phosphoric acid, diphosphate ions, hydrogen phosphate ions, dihydrogen phosphate, linear and/or cyclic oligophosphate ions, linear and/or cyclic polyphosphate ions, phosphonic acids, 1-hydroxyethane-(1,1-diphosphonic acid), aminotrimethylenephosphonic acid, diethylenetriaminepenta(methylenephosphonic acid), ethylenediaminetetra(methylenephosphonic acid), 2-phosphonobutane-1,2,4-tricarboxylic acid, hexafluorophosphate; carboxylic acid anions; silicon-containing anions, preferably silicate anions, in particular hexafluorosilicate, hexafluorotitanate, borate, tetrafluoroborate, acidic boric acid esters and hexafluoroantimonate. The further anions, if present, are preferably present in a concentration between 0.1 g/l and 50 g/l.

Nitrate ions, sulphate ions and/or phosphate ions are particularly preferred in accordance with the invention.

In accordance with a preferred embodiment of the invention, the pH value of the treatment solution is adjusted to values between about 1.5 and 5, preferably from 2 to 4 and in particular from 2.5 to 3.5. The desired pH value can be adjusted by adding hydrogen ions, i.e. by adding an acid and/or a base.

In a preferred embodiment of the invention, the aqueous treatment solution contains at least one organic and/or inorganic acid. The organic acid is preferably selected from the group consisting of citric acid, malonic acid, formic acid, tartaric acid, lactic acid, malic acid, gluconic acid, ascorbic acid, oxalic acid, succinic acid, and adipic acid. When an inorganic acid is used, it is preferably selected from the group consisting of phosphoric acid, polyphosphonic acid, nitric acid, hydrochloric acid and sulfuric acid. In accordance with the invention nitric acid is preferred.

According to another preferred form of the invention, the treatment solution contains weak oxidizing agents, preferably selected from nitrite, amine oxides such as hydroxylamine or hydroxylamine compounds, N-oxides such as m-nitrobenzene sulfonate, nitroguanidine 4-picoline N-oxide, N-methylmorpholine N-oxide and derivatives thereof.

In addition, the treatment solution in accordance with the invention advantageously exhibits a content of volatile organic compounds (VOC), for example alcohol, such as methanol or ethanol, of less than 10% by weight, preferably less than 5% by weight, particularly preferably less than 4% by weight, very particularly preferably less than 3% by weight, in particular from 0.001 to 2% by weight, based on the finished composition. The VOC content can, for example, be determined by GC or GC/MS.

The treatment solution may also contain auxiliary substances, for example selected from the group consisting of polymers, in particular organic polymers, corrosion inhibitors; silicas, in particular colloidal or dispersed silicas; surface-active substances, in particular surfactants; diols, triols, polyols; organic acids, in particular monocarboxylic acids; amines; plastic dispersions; dyes, pigments, in particular carbon black, pigment formers, in particular metallic pigment formers; amino acids, in particular glycine; dispersing auxiliaries. Suitable surfactants are, for example, aliphatic fluorocarbon sulfonates.

Another object of the present invention is a concentrate for the production of a treatment solution in accordance with the invention. The concentrate may be in solid or liquid form and preferably has an active substance concentration which allows the treatment solution in accordance with the invention to be produced by dilution with at least 50% by weight, preferably at least 70% by weight, of water relative to the total weight of the treatment solution. Practical trials have shown that it is appropriate to produce the treatment solution from two concentrates of different composition, as this ensures a longer storage stability of the respective concentrates. A further object of the invention is thus a kit comprising at least 2 concentrates of different composition for the preparation of the treatment solution in accordance with the invention. The concentrates preferably have an active substance concentration which, in combination, makes it possible to prepare the treatment solution according to the invention by diluting it with at least 50% by weight and preferably at least 70% by weight of water.

With the treatment solution in accordance with the invention, workpieces with surfaces made of zinc or zinc alloys can preferably be provided with a conversion coating. In this process, workpieces can be passivated if they are provided with a zinc coating produced by alkaline cyanidic, alkaline cyanide-free or acidic non-cyanidic electrolytic zinc deposition, if they are coated with zinc layers from thermal diffusion processes, or if they are galvanized by means of a melt or if they consist of zinc or a zinc alloy at all. Zinc alloys on the workpiece surfaces can be Zn/Fe, Zn/Ni, Zn/Al and Zn/Co alloys. Furthermore, the reaction solution can also be used to treat workpieces in accordance with the invention on which, in addition to the zinc or zinc alloy surfaces, surfaces are exposed which do not consist of zinc or a zinc alloy, for example iron-containing surfaces such as steel surfaces. These other surfaces can be passivated together with the zinc or zinc alloy surfaces. In principle, it is also possible to use the reaction solution in accordance with the invention to passivate aluminum, aluminum alloy surfaces, iron and iron alloy surfaces, magnesium and magnesium alloy surfaces and surfaces consisting of and/or containing cadmium.

Furthermore, the present invention relates to a process for producing a conversion layer on metallic surfaces, in particular workpieces comprising zinc or zinc alloy layers, in which the workpiece to be treated is brought into contact with a treatment solution in accordance with the invention.

Advantageously, the contacting occurs by immersion, i.e. the workpieces are immersed in the treatment solution contained in a container. The workpieces can either be held on racks and immersed in the reaction solution with the racks, or they can be placed in a drum or centrifuge or on a tray and immersed in the reaction solution with the drum or tray. In an alternative process, the workpieces are also brought into contact with the treatment solution by spray dipping. In another process, the workpieces are also brought into contact with the treatment solution by spraying. In an alternative procedure, the workpieces can also be brought into contact with the treatment solution by flooding. Furthermore, the workpieces can also be sprayed with the treatment solution, for example by means of a nozzle from which a surge of the treatment solution emerges. Another method of treatment is to apply the treatment solution to the workpiece surfaces by brushing, rolling or another application technique. Treatment can take place in conventional facilities in which the workpieces are treated in batches or in continuous flow facilities in which the workpieces are continuously passed through and treated.

One advantage of the method in accordance with the invention is that it can be carried out under similar process conditions as commercial methods with treatment solutions containing cobalt. Thus, it is possible to use the installation technology of the prior art.

In a preferred embodiment of the invention the treatment solution has a bath temperature of about 20 to 100° C., preferably 20 to 80° C., preferably 25 to 60° C., especially preferably 30 to 50° C.

If the workpieces are brought into contact with the treatment solution by immersion, the immersion time is preferably between 5 and 700 seconds, more preferably between 15 and 600 seconds, and in particular between 20 and 240 seconds. Depending on the technique with which the workpieces are brought into contact with the reaction solution, longer or shorter treatment times may also be necessary.

Before contacting the reaction solution, the workpieces to be treated are first cleaned, if necessary. This process step can also be omitted, for example if the workpieces are brought into contact with the reaction solution immediately after electrolytic galvanizing and subsequent rinsing of the galvanizing solution. After completion of the process according to the invention, the workpieces are preferably dried, for example with warm air. In addition, the workpieces can also be rinsed before drying to remove excess reaction solution from the surface.

In a preferred embodiment of the invention the procedure is single-stage. It is also conceivable, however, that the treated object is treated with another treatment solution, such as a sealing solution, before and/or after the application of the conversion coating.

Another object of the present invention is a conversion layer produced by a process as described above.

In an embodiment of the invention, the conversion layer contains compounds of the elements Cr, Si, 0, F, C, H, as well as Zr and/or Ti, even more preferably it consists of these compounds, whereby compounds originating from the metallic surface, for example iron and/or zinc, can also be contained. The weight fractions of the elements can be measured by energy disperse X-ray spectroscopy, EDX.

In practical experiments it was found that the weight percentage of the elements in a zinc surface passivated by the method in accordance with the invention, measured by means of energy disperse X-ray spectroscopy (EDX) at an excitation voltage of 20 kV, is usually in the following ranges: Chromium is usually in the range from 0.05 to 2 wt. %, preferably from 0.1 to 1.5 wt. %, even more preferably from 0.15 to 1.3 wt. %, in particular from 0.2 to 1.0 wt. %, in each case based on the total weight of carbon, oxygen, fluorine, silicon, zirconium, titanium, chromium, iron, zinc. The percentage by weight of silicon is usually in the range from 0.05 to 10 wt. %, preferably from 0.1 to 7 wt. %, even more preferably from 0.2 to 5 wt. %, in particular from 0.3 to 3 wt. %, in each case based on the total weight of carbon, oxygen, fluorine, silicon, zirconium, titanium, chromium, iron, zinc. The percentage by weight of oxygen is usually in the range from 1 to 25 wt. %, preferably from 1.5 to 22 wt. %, even more preferably from 2 to 20 wt. %, in particular from 3 to 15 wt. %, based in each case on the total weight of carbon, oxygen, fluorine, silicon, zirconium, titanium, chromium, iron, zinc. The percentage by weight of fluorine is usually in the range from 0.05 to 3 wt. %, preferably from 0.1 to 2 wt. %, even more preferably from 0.15 to 1.5 wt. %, in particular from 0.2 to 1.0 wt. %, in each case based on the total weight of carbon, oxygen, fluorine, silicon, zirconium, titanium, chromium, iron, zinc. The percentage by weight of zirconium and/or titanium is usually in the range from 0.1 to 5 wt. %, preferably from 0.2 to 4 wt. %, even more preferably from 0.3 to 3 wt. %, in particular from 0.5 to 2.5 wt. %, in each case based on the total weight of carbon, oxygen, fluorine, silicon, zirconium, titanium, chromium, iron, zinc.

In a preferred embodiment of the invention the percentage by weight of zirconium and/or titanium in the conversion layer is higher than the percentage of chromium. This, in combination with the silicon oxide nanoparticles, provides particularly good corrosion protection. The conversion layer may also contain other elements such as Ni, Al, Fe, Co, Cd, for example from the treated surface.

The thickness of the conversion layer, for example, can vary depending on the desired corrosion protection properties. For most applications, it has proven to be convenient to adjust the conversion coating with an average coating thickness from 100 nm to 1000 nm, preferably from 100 nm to 600 nm, and in particular from 150 nm to 400 nm. The coating thickness can be determined by measuring a fracture in the scanning electron microscope.

In accordance with a preferred embodiment of the invention, the conversion coating provides corrosion protection in the salt spray test according to ISO 9227 and/or ASTM B 117-73 without or with heat stress, for example at 120° C. for 24 hours, to an object containing zinc or zinc alloy coatings for more than 100 hours, preferably more than 200 hours and in particular more than 240 hours, until initial attack according to DIN 50961 Chapter 10.

A further advantage of the conversion coating in accordance with the invention is that it can be produced without coloring components. For example, it can show a clear, transparent and essentially colorless and iridescent appearance on zinc. This facilitates an optional selectively set coloring, for example for easier differentiation of components. For example, the treatment solution in accordance with the invention can be used to reliably and easily differentiate between right-handed and left-handed components by means of targeted coloring. This increases process reliability enormously, especially when processing very similar components in large quantities. For this purpose, the conversion coating may contain dyes or pigments.

As mentioned above, the conversion layer in accordance with the invention already provides the treated object with excellent corrosion protection. Therefore, it is not necessary to provide it with an additional layer, for example a sealing layer. Nevertheless, the conversion coating in accordance with the invention is excellently suited as a basis for further inorganic and/or organic layers.

Objects or articles which have a conversion coating in accordance with the invention can therefore be protected permanently and thus particularly advantageously against corrosion. Objects or articles which have a conversion layer in accordance with the invention are also object of the present invention.

Hereafter, the invention is explained in more detail using several non-limiting examples:

Example 1: Preparation of Different Treatment Solutions in Accordance with the Invention

Four treatment solutions in accordance with the invention with the following compositions are produced:

Treatment Solution 1

chromium(III)nitrate solution (74%) 15.0 potassium tetrafluoroborate 0.36 hexafluorocirkonic acid (45%) 14.5 potassium hexafluorozikonate 3.0 hexafluorosilicic acid (35%) 1.5 Acticide MB 2.0 Korantin PP 2.0 Bindzil CC 401 50 g/l

Treatment Solution 2

chromium(III)nitrate solution (74%) 10.0 tetrafluoroboric acid (50%) 0.5 hexafluorocirkonic acid (45%) 17.6 hexafluorosilicic acid (35%) 1.5 Bindzil CC 401 25 g/l

Treatment Solution 3

chromium(III)nitrate solution (74%) 10.0 tetrafluoroboric acid (50%) 0.5 hexafluorocirkonic acid (45%) 17.6 hexafluorosilicic acid (35%) 1.5 Bindzil CC 401 25 oxalic acid 1.5

Treatment Solution 4

chromium(III)nitrate solution (74%) 10.0 tetrafluoroboric acid (50%) 0.5 hexafluorocirkonic acid (45%) 17.6 hexafluorosilicic acid (35%) 1.5 Bindzil CC 401 25 potassium hydrogen difluoride 1.644

Example 2: Treatment of Zinc-Coated Steel Sheets with the Treatment Solutions in Accordance with the Invention

The treatment solutions produced in example 1 are used to treat zinc-coated steel sheets. The steel sheets were produced alkaline cyanide-free by use of commercially available SurTec® 704. The treatment parameters are set as follows:

Treatment solution 1 2 3 4 Treatment temperature 40° C. 40° C. 40° C. 40° C. pH-value 3.2 3.0 3.0 3.0 Tretment duration 2 min 1 min 120 s 120 s

Example 3: Determination of the Corrosion Protection Properties of the Treated Steel Sheets

The treated steel sheets obtained in example 2 are examined for their corrosion protection properties by means of a salt spray test according to ISO 9227 and ASTM B 117-73. As a result, the following results are obtained:

Treatment solution 1 2 3 4 Heat stress 120° C. 120° C. 120° C. 120° C. Stress duration 24 h 24 h 24 h 24 h Corrosion protection >240 h >120 h >336 h >336 h until initial attack

The results show that the conversion layers in accordance with the invention allow zinc-coated steel sheets to be provided with a corrosion protection in the salt spray test according to DIN EN ISO 9227 and/or ASTM B 117-73 even at a heat load of 120° C. for 24 hours until the first attack according to DIN 50961 Chapter 10 of up to more than 300 hours.

Example 4: Determination of the Element Composition of the Conversion Coating Produced with Treatment Solution 3

The composition is measured by energy disperse X-ray spectroscopy (EDX). The result of the measurement is shown in the following table.

TABLE Result of energy disperse X-ray spectroscopy (EDX) of a conversioncoating produced using the treatment solution 3 in accordance with the invention. Elem Wt % At % C K 6.36 19.80 O K 12.75 29.78 F K 0.61 1.19 SiK 4.57 6.09 ZrL 1.97 0.81 CrK 0.51 0.37 FeK 1.03 0.69 ZnK 72.20 41.28 Total 100.00 100.00

It turns out that the percentage by weight of zirconium is higher than the percentage by weight of chromium.

Example 5: Weight Percentages of the Elements of Different Conversion Coatings in Accordance with the Invention

The weight percentage of the elements on a passivated zinc surface in accordance with the invention was measured by means of energy disperse x-ray spectroscopy (EDX) at an excitation voltage of 20 kV and evaluated by means of EDAX Genesis V5.11.

The following table lists the measured weight percentages of the elements of 8 conversion coatings in accordance with the invention.

Element, W % KS 1 KS 2 KS 3 KS 4 KS 5 KS 6 KS 7 KS 8 C K 7.61 7.85 6.76 6.40 4.56 4.05 7.88 7.38 O K 8.74 7.97 7.29 6.43 4.92 4.79 9.19 11.14 F K 0.47 0.30 0.36 0.34 0.32 0.60 n.a. n.a. SiK 3.06 2.72 2.37 1.44 0.50 0.80 3.19 4.52 ZrL 1.60 1.33 1.46 1.36 1.30 1.61 1.60 1.54 CrK 0.34 0.41 0.32 0.67 0.47 0.39 0.46 0.43 FeK 0.41 0.46 0.96 0.50 0.96 0.80 1.26 1.16 ZnK 77.78 78.96 80.48 82.86 81.96 86.95 76.41 73.84 Total 100.000 100.000 100.000 100.000 100.000 100.000 100.000 100.000

FIG. 1 shows the cross-section of the conversion coating produced with treatment solution 3. The thickness of the conversion coating was measured at 10 different points using mean value formation. The average coating thickness is about 300 nm.

FIG. 2 shows the cross-section of the conversion coating produced with treatment solution 4. The thickness of the conversion coating was measured at 10 different points using mean value formation. The mean coating thickness is about 250 nm.

Elem Wt % At % C K 6.36 19.80 O K 12.75 29.78 F K 0.61 1.19 SiK 4.57 6.09 ZrL 1.97 0.81 CrK 0.51 0.37 FeK 1.03 0.69 ZnK 72.20 41.28 Total 100.00 100.00

While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive. It will be understood that changes and modifications may be made by those of ordinary skill within the scope of the following claims. In particular, the present invention covers further embodiments with any combination of features from different embodiments described above and below. Additionally, statements made herein characterizing the invention refer to an embodiment of the invention and not necessarily all embodiments.

The terms used in the claims should be construed to have the broadest reasonable interpretation consistent with the foregoing description. For example, the use of the article “a” or “the” in introducing an element should not be interpreted as being exclusive of a plurality of elements. Likewise, the recitation of “or” should be interpreted as being inclusive, such that the recitation of “A or B” is not exclusive of “A and B,” unless it is clear from the context or the foregoing description that only one of A and B is intended. Further, the recitation of “at least one of A, B and C” should be interpreted as one or more of a group of elements consisting of A, B and C, and should not be interpreted as requiring at least one of each of the listed elements A, B and C, regardless of whether A, B and C are related as categories or otherwise. Moreover, the recitation of “A, B and/or C” or “at least one of A, B or C” should be interpreted as including any singular entity from the listed elements, e.g., A, any subset from the listed elements, e.g., A and B, or the entire list of elements A, B and C. 

1. An aqueous treatment solution for producing conversion coatings on metal surfaces, in particular on zinc or zinc alloy surfaces, the treatment solution containing: Cr(III) ions in an amount of 0.1 g/l to 8.0 g/l; zirconium and/or titanium ions in an amount of 0.1 g/l to 15 g/l; organosilane-modified silicon oxide nanoparticles in an amount of 0.1 g/l to 50 g/l; and fluoride ions in an amount of 0.1 g/l to 15 g/l.
 2. The treatment solution according to claim 1, wherein the treatment solution has a chromium(VI) ion content of less than 1 mg/l and/or a cobalt ion content of less than 10 mg/l and/or a Ni ion content of less than 10 mg/l.
 3. The treatment solution according to claim 1, wherein the organosilane-modified silicon oxide nanoparticles have a core of agglomerated silicon oxide, on a surface of which organic silane compounds are arranged.
 4. The treatment solution according to claim 1, further comprising oxygen atoms on a surface of the silicon oxide nanoparticles covalently bonded to silicon atoms of an epoxy, amido, ureido, amino, ester, mercapto, and/or isocyanate silane.
 5. The treatment solution according to claim 1, wherein the organosilane-modified silicon oxide nanoparticles have an average particle size of 5 to 50 nm.
 6. The treatment solution according to claim 1, wherein a molar ratio between Cr(III) ions and zirconium and/or titanium ions in the treatment solution is from 0.1 to 2.0.
 7. The treatment solution according to claim 1, wherein a pH of the treatment solution is between about 1.5 and
 5. 8. The treatment solution according to claim 1, wherein a volatile organic compound (VOC) content of the treatment solution is less than 10% by weight.
 9. A concentrate for the preparation of the treatment solution according to claim 1, the concentrate having an active substance concentration which allows a preparation of the treatment solution by dilution with at least 50% by weight of water, based on a total weight of the treatment solution.
 10. A kit comprising at least two concentrates according to claim 9, having a different composition.
 11. A process for producing a conversion coating on workpieces having metallic surfaces, in particular zinc or zinc alloy layers, the process comprising: bringing a workpiece to be treated into contact with the treatment solution according to claim
 1. 12. A conversion coating produced by the process according to claim
 11. 13. The conversion coating according to claim 12, wherein the conversion coating contains compounds of elements Cr, Si, O, F, C, H and Zr, and/or Ti.
 14. The conversion coating according to claim 12, wherein a weight percentage of zirconium and/or titanium in the conversion coating is higher than a weight percentage of chromium.
 15. The conversion coating according to claim 12, the conversion coating having an average layer thickness of 100 nm to 1000 nm.
 16. The concentrate according to claim 9, the concentrate having an active substance concentration which allows a preparation of the treatment solution by dilution with at least at least 70% by weight of water, based on the total weight of the treatment solution. 